The goal of this proposal is to understand the structural basis of specificity and mechanism in C-type (Ca/2+-dependent) lectins that function in the innate and adaptive immune responses, using x-ray crystallography as the principal tool. 1) Mannose-binding proteins (MBPs) recognize oligosaccharide structures present on pathogenic surfaces and trigger killing of these organisms by activating the complement system. The ability of MBPs to distinguish foreign from self depends on high avidity, multivalent binding to pathogenic cells but not endogenous glycoconjugates. As multivalent binding bends on spacing of binding sites and recognition of ligands presented in particular orientations, structures of MBP oligomers and MBP mutants that bind to ligands in alternative orientations will be examined. 2) The mannose receptor or macrophages and dendritic cell endocytoses pathogens and potentially harmful glycoconjugates, leading to their destruction and processing for antigen presentation. Structures of a single carbohydrate-recognition domain (CRD) will be determined in complex with ligands to probe the unique features of ligand recognition by this receptor. The structure of a receptor fragment containing two CRDs will be determined in order to visualize the spacing of binding sites and its effects on multivalenty. Ca/2+ binding to CRD-4 will be examined by fluorescence and crystallographic methods, to probe the molecular basis of the pH-induced loss of Ca/2+ and sugar affinity in the endosome. Mutants of MBP engineered to mimic the pH transition of endocytic receptors such as the mannose receptor, will also be examined. 3) The selectin cell adhesion molecules mediate the first step in leukocyte targeting to lymphoid tissues and sites of inflammation. The structures of MBP/selectin chimeras that mimic the binding of selectins will be determined in complex with selectin ligands, as will the structure of an L-selectin fragment containing the lectin and EGF-like domains.